The present invention relates to improvements in apparatus for ascertaining and/or regulating the amounts of radiation for the making of images with X-rays, gamma rays or analogous radiation. More particularly, the invention relates to improvements in so-called dosimeters which are apparatus serving to measure the doses or the effects of doses of ionizing radiation. Still more particularly, the invention relates to improvements in dosimeters which can be used with advantage in mammography.
A dosimeter is needed in order to automatically regulate the exposure of mamma during the making of X-ray images or similar images in a mammograph. The dosimeter determines the necessary and/or maximum permissible amounts of radiation. Many presently known dosimeters constitute ionization chambers which are filled with air or another gas. Other types of presently known dosimeters include semiconductor measuring cells which are sensitive to X-rays.
A drawback of presently known dosimeters, especially those which are used in mammography, is that they absorb a substantial percentage of X-rays in the voltage range between 25 and 50 kilovolts. Consequently, such dosimeters are imaged on the image carrier of the mammograph provided that they are installed between the source of X-rays and the image carrier (e.g., an X-ray film or another record carrier whose image can be rendered visible by resorting to toner particles or the like). The placing of a conventional dosimeter behind the image carrier is even more undesirable because (depending on the thickness of the object to be X-rayed, on the nature of the filter means for the X-ray tube and/or on the selected voltage of the tube) more than 80 percent and, under unfavorable circumstances, in excess of 90 percent of radiation is absorbed in the chamber. This distorts and renders highly unreliable the measurements which are carried out by the dosimeter, i.e., the doses of radiation to which an object is exposed can deviate very substantially from the actual doses. In other words, the resuls of measurements are much more accurate if the dosimeter is placed between the radiation source and the image carrying sheet, such as an X-ray film or the like. As stated above, conventional dosimeters are not particularly suited for such mode of installation because their images are clearly visible on the image carrier.